title: Kinetic and electrochemical studies of the oxidative addition of demanding organic halides to Pd(0): the efficiency of polyphosphane ligands in low palladium loading cross-couplings decrypted.
abstract: Oxidative addition (OA) of organic halides to palladium(0) species is a fundamental reaction step which initiates the C-C bond formation catalytic processes typical of Pd(0)/Pd(II) chemistry. The use of structurally congested polyphosphane ligands in palladium-catalyzed C-C bond formation has generated very high turnover numbers (TONs) in topical reactions such as Heck, Suzuki, Sonogashira couplings, and direct sp(2)C-H functionalization. Herein, the OA of aryl bromides to Pd(0) complexes stabilized by ferrocenylpolyphosphane ligands L1 (tetraphosphane), L2 (triphosphane), and L3 (diphosphane) is considered. The investigation of kinetic constants for the addition of Ph-Br to Pd(0) intermediates (generated by electrochemical reduction of Pd(II) complexes coordinated by L1-L3) is reported. Thus, in the OA of halides to the Pd(0) complex coordinated by L1 the series of rate constants kapp is found (mol(-1) L s(-1)): kapp(Ph-Br) = 0.48 > kapp(ClCH2-Cl) = 0.25 ≫ kapp(p-MeC6H4-Br) = 0.08 ≈ kapp(o-MeC6H4-Br) = 0.07 ≫ kapp(Ph-Cl). Kinetic measurements clarify the influence that the presence of four, three, or two phosphorus atoms in the coordination sphere of Pd has on OA. The presence of supplementary phosphorus atoms in L1 and L2 unambiguously stabilizes Pd(0) species and thus slows down the OA of Ph-Br to Pd(0) of about 2 orders of magnitude compared to the diphosphane L3. The electrosynthesis of the complexes resulting from the OA of organic halides to [Pd(0)/L] is easily performed and show the concurrent OA to Pd(0) of the sp(3)C-Cl bond of dichloromethane solvent. The resulting unstable Pd/alkyl complex is characterized by NMR and single crystal X-ray structure. We additionally observed the perfect stereoselectivity of the OA reactions which is induced by the tetraphosphane ligand L1. Altogether, a clearer picture of the general effects of congested polydentate ligands on the OA of organic halides to Pd(0) is given.
authors.affiliation: Institut de Chimie Moléculaire de l'Université de Bourgogne, UMR-CNRS 6302, Université de Bourgogne , 9 Avenue Alain Savary, 21078 Dijon, France.
authors.forename: Veronika A
authors.initials: VA
authors.lastname: Zinovyeva
authors.forename: Sophal
authors.initials: S
authors.lastname: Mom
authors.forename: Sophie
authors.initials: S
authors.lastname: Fournier
authors.forename: Charles H
authors.initials: CH
authors.lastname: Devillers
authors.forename: Hélène
authors.initials: H
authors.lastname: Cattey
authors.forename: Henri
authors.initials: H
authors.lastname: Doucet
authors.forename: Jean-Cyrille
authors.initials: JC
authors.lastname: Hierso
authors.forename: Dominique
authors.initials: D
authors.lastname: Lucas